Keep Advanced Settings quick ranges locked in the time picker

## Summary

Quick ranges from timepicker:quickRanges are no longer copied into user storage. The time range selector merges locked admin ranges with user-created presets. Only user-created presets can be deleted. Advanced Settings already describes the setting. It does not say those ranges stay locked in the picker.

**Why this needs docs:** The setting page already defines these ranges. A one-line lock note would match the new delete behavior.

## Resources

- PR [#283527](https://github.com/elastic/kibana/pull/283527) — [Data][DateRangePicker] Prevent deletion of quick ranges configured in Stack Management
- Product issue: https://github.com/elastic/kibana/issues/283462

## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.6.0 |
| **Serverless** | Aug 17–Aug 21 |
| **Feature flag** | None — active by default |

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## Suggested edits

[Kibana advanced settings](https://www.elastic.co/docs/reference/kibana/advanced-settings) > timepicker:quickRanges
- **What the docs say:** The list of ranges to show in the Quick section of the time filter. This must be an array of objects, with each object containing from, to (refer to accepted formats), and display (the title to be displayed).
- **What to add:** Add that ranges from this setting cannot be deleted in the time range selector. Only user-created presets are deletable. Applies from 9.6.0 and in serverless.